1. Field of the Invention
The field of the present invention relates in general to semiconductor processing. More particularly, the field of the invention relates to systems and methods for selectively removing unwanted material from a surface of a semiconductor wafer, especially those residues left after photoresist ashing and/or reactive ion etching processes.
2. Description of the Prior Art
Many of the processes used in submicron scale integrated circuit fabrication involving thin film patterning (etching) or doping of semiconductor wafers leave chemical residues embedded in or on the surface of the remaining photoresist, or other areas on the sidewalls of features or the top surface of the wafer. Often these residues are insoluble or form insoluble compounds in the typical photoresist removal process called ashing (in which active species from oxygen discharges oxidize the remaining photoresist) and are not removed by the water rinsing which follows. Such residues, if left on the surface of the wafer after photoresist removal, could cause degradation of the performance of the devices or viability of the integrated circuit and therefore should be removed. Removal of these residues can be difficult and has historically been done by immersion in wet chemical baths containing either acidic or caustic chemicals. Often such chemicals are expensive and may require special handling in use and in disposal as toxic materials.
There are some residues which are easily removed including those containing silicon, carbon and halogens which remain after polysilicon gate etch or silicon dioxide contact etch processes. However, removal of the residues and polymers from the surfaces of the device structures after other processes is often difficult and may require such aggressive liquid treatments that materials on the surfaces of the device may also be etched or undesirable contaminants may be left on the wafer surface causing performance degradation.
Often processes either involving metal etching or etching down to a metal stop layer leave difficult residues. Also, surfaces of device structures can be contaminated by particulates as a result of such wet chemical treatment. One such residue which may be difficult to remove is that found after etch patterning of an aluminum layer. In this case the photoresist ashing process environment (almost always done immediately after etching in the same tool to avoid corrosion by atmospheric moisture) is very strongly oxidizing causing the aluminum, silicon, and carbon containing residues in the photoresist to transform to a mixture including aluminum oxide which is very resistant to chemical-attack. Another such residual layer is usually found after etching of silicon dioxide to create vias. (Vias are the connections made from one level of metal lines to the adjacent level(s).) At the last stage of such an etching process there can be sputtering of the metal underlying the insulating layer causing formation of metal containing residues on the sidewalls of the vias.
There are also other etching or implantation processes which create metal or silicon containing residues or other hard polymeric materials on the surface that are very hard to remove by wet chemical processes. In most cases plasma-activated gas streams containing oxygen may be ineffective in ashing or making soluble such remnant materials on the wafer surface. Usually, exposure of such materials to plasma-activated gaseous streams containing oxygen, which is needed for ashing the photoresist, causes oxidation of such residues which forms even more difficult to remove materials. Other such residual materials might include some of the major dopants for semiconductors such as: boron; phosphorus; other metal layers, such as titaniumxe2x80x94which is used for various purposes in the integrated circuit; silicon carbide; highly cross-linked or diamond-like carbon; and recently, copper has been used in integrated circuits, and whose oxides and residues need to be removed from insulator surfaces.
It is desirable therefore to find process techniques other than wet etching which enable such residues to be removed without causing excessive etching of other materials exposed on the wafer surface. Heretofore, dry chemical removal processes employing plasma sources separated from the wafer processing region, using flows of reactive species from that source to the wafer processing region have not been successful at removing such residues. What is needed is a system and method for selectively removing unwanted materials from a semiconductor wafer.
Aspects of the invention provide a system and method for selectively removing unwanted material from a surface of a semiconductor wafer without causing damage to or etching of underlying portions of the semiconductor.
One embodiment of the invention includes using reactive species from a plasma source to facilitate the removal of residues remaining after metal etching on a silicon wafer, where the gases employed in creating the plasma include little or no oxygen. This includes flowing a gaseous mixture with little or no oxygen to a plasma source where reactive species are created which then flow to a semiconductor wafer causing volatile or soluble species to be formed from the metal-containing residues on the semiconductor wafer. In this embodiment the oxygen flow is less than or equal to 2% of the total gas flow (and may be zero), or less than one third of the flow of hydrogen (hydrogen containing, and/or other reducing gases), whichever is less. The flow of the hydrogen, hydrogen-containing gases, and/or other reducing gases, comprises greater than 2% of the total gas flow. The hydrogen may be in molecular form as H2 or other hydrogen containing gases such as methane or other hydrocarbons, ammonia or gaseous amines, water vapor or alcohols. These gases produce reactive species in the plasma which, when caused to flow to the wafer, perform selective removal (or facilitate such removal in succeeding steps) of residues which may not be removed were oxygen to be a larger component of the mixture. The reducing nature of the reactive species in this case tends to disfavor the creation of metallic oxides which are resistant to solution in water rinses. Examples of critical layers on a wafer surface which might need to be preserved from erosion or damage might include but are not limited to anti-reflective coating layers (ARC) or barrier layers such as titanium nitride or titanium.)
Suitable plasma sources useful in embodiments of the invention include, but are not limited to, an inductively coupled plasma source which may or may not employ a partial electrostatic shield (Faraday shield), as described in U.S. Pat. No. 5,534,231, downstream-type plasma-source based tools such as are commonly used for ashing photoresist, which employ a non-resonant type (waveguide based) of microwave (typically 2.45 GHz) plasma source, a resonant cavity type microwave plasma source (Evenson), or other types commonly used for plasma-based processes including RF capacitively coupled sources, a resonant microwave based source, possibly including so-called xe2x80x9cECRxe2x80x9d sources, as well as UHF sources which use antenna(e) as launchers of electromagnetic energy with frequency at or above 80 MHz into the plasma.
One exemplary embodiment of the invention includes a method and system for removal of difficult residues such as those formed after the etching of aluminum, or residues formed after the resist ashing following aluminum etching. In this embodiment a processing step is performed in which a combination of hydrogen and carbon tetrafluoride gases (but with substantially no oxygen) is injected into an inductively coupled plasma source with a partial Faraday shield to form neutral and charged species to which the wafer is exposed. The wafer is held at relatively low temperature (below 100 Celsius), causing attack (leading to removal during the water rinse following processing) of the metal containing residues while leaving the critical material layers on the surface of the wafer substantially unharmed. Such treatment provides a slight degree of energetic ion bombardment (of the order or less than 80 microamperes/square centimeter) to help destroy chemical and physical bonds of the residues to the wafer surface.
In other embodiments of the invention, the residue removal process or process step or steps facilitating the removal of such residues comprise a subset of the totality of process steps carried out sequentially in the same processing system. In one aspect of these embodiments, the residue removal process is one or more of the contiguous steps of the total process such that there is no inactive interval between steps. Other steps in the total process may employ substantial amounts of oxygen gas fed to the plasma source in order to cause different effects on the materials exposed on the wafer. These steps would not substantially interfere with the reducing step in the process wherein there is little or no oxygen employed. In yet another aspect of the invention, residual oxygen from steps preceding low/no oxygen step(s) is substantially pumped out of the chamber prior to these step(s) allowing the process step(s) to avoid residual oxygen and/or its products.
In other embodiments of the invention processing steps may be done in non-contiguous time steps such that there is a time interval between steps, or other steps are done in a separate reactor chamber in the same processing system.
Still other embodiments of the invention include the use of hydrogen or hydrogen containing gases and/or halogenated gases, with substantially no oxygen, as principal constituent(s) to reduce (or otherwise react with) residues and polymers on a wafer for post-treatment following a dielectric etching step called xe2x80x9cviaxe2x80x9d etch. In these embodiments, the residue removal process may be done prior to photoresist ashing in a photoresist ashing tool or an etching tool. Hydrogen containing gases useful in these embodiments include but are not limited to hydrocarbons, ammonia, water vapor or alcohols or mixtures of hydrogen in inert gases such as noble gases or nitrogen, partially fluorinated hydrocarbons, difluoromethane (CH2F2), other fluorocarbons (such as CF4, C2F6, . . . ), SF6, NF3 or F2 or mixtures of other halogenated gases such as freon gases. Very small amounts of oxygen may also be used, of the order of two percent or less of the total gas flow to none at all or any amount within this range.
The ions from the plasma source are typically important in this process since they help promote the chemical reaction of the neutral activated species with the residues. In one embodiment a source of RF power is applied to the wafer holding pedestal, or other means to capacitively couple RF energy into the plasma, to cause there to be a sheath adjacent to the wafer surface such that as the ions flow towards the wafer they accelerate to the wafer surface. In this embodiment the plasma source may be separated from the wafer yet be close enough that the ions flow to the wafer in sufficient quantity to help promote chemical reactions with residue on the wafer.
In other embodiments of the invention where residues are to be removed which do not require ion bombardment, the source region and wafer processing region may be distinct and separated by a distance of up to 50 centimeters. Typically the residue or polymer removal processes in these embodiments are predominantly isotropic processes wherein sputtering and ion-assisted etching processes may be present but are not the dominant mechanisms. The etching or alteration of materials on the wafer surface in these embodiments is mainly by chemical reactions of reactive neutral species produced in the plasma source.
In another embodiment of the invention, a non-resonant microwave source is used to generate a plasma. In one particular embodiment, the microwave source uses an insulating cylindrical tube which is evacuated and through which flows the appropriate gas mixture in the proper pressure. The tube passes through a resonant microwave cavity or waveguide, either perpendicular to the walls of the guide or along its length. The cavity or waveguide is typically several centimeters to tens of centimeters in width or diameter for the commonly used microwave excitation frequency of 2.45 GigaHertz, but may be larger or smaller depending on the frequency of the microwave power utilized.